I would argue almost every living thing is intelligent. The "world" is an incredibly complex thing and every organism has to deal with that. The sensory inputs that any organism has to deal with are incredible. And every living thing deals with it amazingly, superbly well. How do you know? They, and their species, are alive. That's it. That's the test.

There are a lot of philosophical and scientific lines to be drawn between single-cell organisms, multi-cellular organisms, plants, fungi, and on up. They're all debatable. Personally, I draw the "intelligence" line in kingdom Animalia, simply because these are highly complex organisms with a central nervous system that can react and respond to the environment around them immediately.

These organisms are out in the world, processing it and reacting to it. There are some amazing studies of "personality" in things as "low" as water striders. If you break up personality into things as simple as being aggressive or being passive, you see those personalities in water striders. This is a complex neurological arrangement determining how a little lowly insects acts. This is a result of observing and processing information about the world and "deciding" a way to deal with it. That's fucking smart. And they change, depending on situation. They can assess if they can be bullies or not, and act accordingly.

Keep working your way up, inter- and intra-specific interaction become even more complex. The goal of an organism is to be alive (and in the process, reproduce). To stay alive you have to be smart. That's it. You have to, on some level, be smart enough to stay alive (when you have a choice about it.)

Animals are smart. Even water striders are not little organic robots. Humans got really smart for a number of debatable reasons, but every other animal alive is also smart in their own way with their own senses and abilities. They're alive because they're neurologically complex and "smart." If they weren't, they'd be dead.